U.S. Pat. No. 6,050,913 discloses a control circuit for adjusting the tension of the belt part of a belt transmission. The belt part has a drive end and an output end. Sensors detect various operating parameters of the vehicle. The known control circuit includes a circuit component which computes the value for the transmission ratio by division of the primary rpm by the secondary rpm. The value computed in this way is needed by the control circuit for additional computations and therefore, for example, for the control of the tension of the belt part or for the adjustment of the transmission ratio. In this way, a value representing the transmission ratio is computed from the signals for the primary rpm of the drive end and the secondary rpm of the output end which signals are transmitted to the control circuit. However, this value need not correspond in each case to the actual transmission ratio.
With the invention, it is advantageously avoided that the signal, which represents the transmission ratio, is still computed from the instantaneous primary rpm and the secondary rpm when such a computation is unsatisfactory or would lead to results deviating greatly from the actual transmission ratio. In order to avoid the output of a signal for the transmission ratio which is not suitable for further processing, check means are advantageously provided which generate a fault signal which indicates whether a value for the transmission ratio, which is computed from the detected instantaneous values for the primary rpm and the secondary rpm, is to be outputted as the signal which represents the transmission ratio. This fault signal is dependent on the instantaneous primary rpm and/or the instantaneous secondary rpm.
In an advantageous embodiment, it is provided that the signal, which represents the transmission ratio, is read out of a read-only-memory or a stored characteristic line, in the event that the fault signal indicates that the value for the transmission ratio, which is computed from the instantaneously detected values for the primary rpm and the secondary rpm, should not be outputted as the signal representing the transmission ratio.
It is especially advantageous when the fault signal is generated in dependence upon a comparison of the instantaneous rpm and/or the secondary rpm to pregivable parameter ranges.
In this way, especially the rpm ranges having low rpms are precluded for the computation for which a computation of the transmission ratio from the instantaneous detected values for the primary rpm and the secondary ram could lead to results deviating greatly from the actual transmission ratio.
In an especially advantageous embodiment, the value for the transmission ratio, which is computed from the instantaneously detected values for the primary rpm and the secondary rpm, is outputted as the signal, which represents the transmission ratio, provided the instantaneous primary rpm lies within a third parameter range and, simultaneously, a value, which is computed from the instantaneous primary rpm, for the amount of a gradient of the primary rpm lies within a further pregivable parameter range. Accordingly, if the primary rpm would fall within a transition range, in which the computation of the transmission ratio only leads to an unsatisfactory result when the primary rpm changes greatly over time, then, in dependence upon the time-dependent change of the primary rpm, a decision is made as to whether the transmission ratio can still be computed from the values for the primary rpm and the secondary rpm.